Maths Olympiad Prep

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Geometry Difficulty 7.3 National olympiad, round 2 Prove it

ABCDABCD is a trapezoid with ABCDAB || CD. There are two circles ω1\omega_1 and ω2\omega_2 is the trapezoid such that ω1\omega_1 is tangent to DADA, ABAB, BCBC and ω2\omega_2 is tangent to BCBC, CDCD, DADA. Let l1l_1 be a line passing through AA and tangent to ω2\omega_2(other than ADAD), Let l2l_2 be a line passing through CC and tangent to ω1\omega_1 (other than CBCB).

Prove that l1l2l_1 || l_2.

Solution

1. Identify the centers of the circles:
- Let the center of ω1\omega_1 be II.
- Let the center of ω2\omega_2 be EE.
- Note that ω1\omega_1 is tangent to DADA, ABAB, and BCBC, and ω2\omega_2 is tangent to BCBC, CDCD, and DADA.

2. Define the point of intersection:
- Let KK be the point of intersection of ADAD and BCBC.

3. Understand the homothety:
- The center EE' of ω1\omega_1 is the image of the center EE of the excircle of DCK\triangle DCK corresponding to the edge DCDC through a homothety centered at KK with ratio KAKD\frac{KA}{KD}.

4. Prove similarity of triangles:
- Triangles KDIKDI and KCEKCE are similar. This can be shown by noting that both triangles share angle DKI\angle DKI and KID=KEC\angle KID = \angle KEC because II and EE are centers of circles tangent to the same lines.

5. Use the similarity to find angles:
- Since KA/KD=KE/KEKA/KD = KE'/KE, the triangles KIAKIA and KCEKCE' are similar. Hence, KIAECK\angle KIA \equiv \angle E'CK.

6. Calculate the angles:
- We have DAI=DKI2+AIK=DKI2+ECK\angle DAI = \frac{\angle DKI}{2} + \angle AIK = \frac{\angle DKI}{2} + \angle E'CK.
- Therefore, 2DAI=DKI+2KCE2\angle DAI = \angle DKI + 2\angle KCE'.

7. Use the property of tangents:
- The angle between tangents from a point to a circle is twice the angle between a tangent and the line joining that point to the center of that circle.
- Hence, the angle between BKBK and 2\ell_2 is 2KCE2\angle KCE', and the angle between KDKD and 1\ell_1 is 2DAI2\angle DAI.

8. Conclude parallelism:
- From the equation 2DAI=DKI+2KCE2\angle DAI = \angle DKI + 2\angle KCE', we can deduce that 1\ell_1 and 2\ell_2 intersect KCKC at the same angles, hence they are parallel.

12 \boxed{\ell_1 \parallel \ell_2}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.